The double-acting piston was an important improvement for the steam engine.
The steam piston for this engine still reciprocated relatively slowly. For mechanical
mine water pumping with the early engines, the upward and downward piston strokes
took different lengths of time, but it really didn't matter that the reciprocating
was uneven for pumping water. For operating factory machinery, this was not
acceptable, and the double-acting engine described here operated smoothly and
identically in both directions.

In this engine, both the upper and lower cylinder sections are connected
to the condenser and boiler using valves. The engine beam is just visible
at the top of the photo. The right end of the beam is connected by an
arm to a sun and planet gear system that turns the large engine wheel.
The engine wheel is in the rear of the photo. This large engine wheel
has teeth on the outside edge to permit it to turn machinery in the factory.

This photo at left is taken looking downward along the safety railing
shown above, near the engine wheel. (The spokes of the engine wheel can be seen on the
left edge of the left photo.) The sun gear is connected to the shaft of the engine
wheel. The planet gear is connected to the arm that runs up to the beam.
The sun and planet gear is explained below.

This engine uses two identical strokes. Note that the valve mechanisms
of Figure 1 on the upper and lower sections of the piston are identical.
There are two pipes to the left of the steam cylinder connecting the upper
and lower valves, (only one pipe is drawn in the cross section of Fig
1). These two pipes enable both cylinder ends (above and below the piston)
to alternately be connected with the boiler and the condenser. When one
end is connected to the boiler, the other end is connected to the condenser.
Thus the upward and downward stokes are identical. This also means that
the beam can be balanced since the piston does equal work on both motions.
Note that Figure 1 shows a cast iron beam instead of the wooden beam used
in Watt's earlier pumps.

Another change that this engine incorporates is the elimination of the
chain connecting the beam and the steam piston rod. This is done with
a mechanism called the parallel motion mechanism. This is the invention
that Watt told his son made him the most proud, even though the public
remembers Watt for his other contributions. This mechanism makes it possible
to match the linear motion of the piston with the arc motion of the beam.
It is also rigid enough to provide pushing as well as pulling by the piston.

Note the speed governor in the center of Figure 1. The speed governor
is connected to the engine wheel (not shown) though a chain and pulley
at the base of the governor. The chain spins the governor when the engine
wheel turns. When the engine starts to speed up, the governor spins faster,
and centrifugal force will force the balls to rise as shown by the lighter
outlines in Figure 2. The position of the balls is an indication of the
rotational speed. Watt did not invent this device; the device was already
in use for other rotating machinery. Watt invented the method to feed
this speed signal back to the steam throttle to control the rate steam
is admitted to the cylinder.

When the balls spread, the lever n-f-g at point n will
be pulled down, causing point g to rise. In Figure 1, this motion
will tend to close the throttle valve, decreasing steam flow into the
piston, and slowing the engine. A slowing of the engine will create the
opposite effect on the governor and throttle.

Watt invented a sun and planet gear system to be connected to the beam.
The crankshaft (like a bicycle pedal) achieves the same basic motion and
is much simpler than the sun and planet system employed here. The crankshaft
already was invented and patented by others but wasn't used here anyway.
(Watt said that a machinist stole crankshaft idea from him after he had
described it, but was too busy to worry about litigation). The reason
that the sun and planet is superior for this application is that a crank
will only turn a wheel once per stroke of the piston. This device, although
more complex, can turn a wheel faster. Although this device isn't used
very much any more because of it's complexity and because we now use pistons
that reciprocate very rapidly, the device is significant for getting slow
linear motion converted to rotative motion at acceptable speeds.

The large wheel in Figure 3 is the engine wheel. This wheel was used
as the driving pulley for a large belt. The belt often powered a pulley
system suspended from the factory ceiling. The pulley system distributed
rotative power throughout the factory, mill, etc.